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anawan Supawannawiwat, M.D., Chottiwat Tansirisithikul, M.D., BunpotSitthinamsuwan, M.D.,M.Sc.
Division of Neurosurgery, Department of Surgery, Faculty of Medicine Siriraj Hospital, MahidolUniversity, Bangkok 10700, ailand
Temporal Bone Landmarks of the Transverse-
sigmoid Sinus Junction: An Anatomical Study in
Dried Human Skulls
ABSTRACT
Objective: To investigate the accuracy in localization of the anterosuperior margin of TSSJ by using the intersection
point between the squamosal and parietomastoid sutures (A point) and the intersection of the squamosal suture
and supramastoid crest (B point) as bony landmarks.
Materials and Methods: e A and B points were marked on the inner surface of a skull by using the transillumination
technique. e anatomical relationship between the projected A point, B point, and groove of TSSJ was investigated
in 60 dried ai human skulls (120 sides).
Results: Of the 120 sides, the projected A points were located exactly on the anterosuperior margin of the TSSJ in
38 (31.7%) instances and adjacent (above and below) the anterosuperiormargin in 82 (68.3%) cases. Of the 118
sides with identiable supramastoid crests, the projected B points were located precisely on the anterosuperior
margin of TSSJ in 60 (50.8%) cases and above the anterosuperior margin of the TSSJ in 57 (48.3%) cases.Hence,
the projected B point was a more reliable bony landmark for localizing the anterosuperior margin of the TSSJ when
compared with the projected A point (p = 0.003, OR 2.2, and 95% CI =1.3-3.8).
Conclusion: e B point is a more reliable temporal bone landmark for localization of the TSSJ than the A point.
In temporal craniotomy, an initial burr hole at the B point is relatively safe and carries a very low risk of inadvertent
venous sinus injury.
Keywords: Relationship; transverse-sigmoid sinus junction; squamosal suture; parietomastoid suture; supramastoidcrest;
temporal craniotomy; middle cranial fossa (Siriraj Med J 2021; 73: 738-743)
Corresponding author: Chottiwat Tansirisithikul
E-mail: tansirichok@hotmail.co.th
Received 21 January 2021 Revised 20 April 2021 Accepted 31 May 2021
ORCID ID: https://orcid.org/0000-0001-6562-0671
http://dx.doi.org/10.33192/Smj.2021.95
INTRODUCTION
In neurosurgical practice, temporal craniotomy
is one of the most common surgical approaches for
dealing with lesions that involve the middle cranial
fossa. is procedure is also the key component for
more aggressive lateral skull base approaches such asthe
transpetrosalapproach. e posterior boundary of this
approach is dened by the transverse-sigmoid sinus
junction (TSSJ). In order to maximize craniotomy size
and to avoid inadvertent venous sinus injury, localization
of this major venous sinus is crucial during planning for
craniotomy.
1-9
Although the neuronavigation system is
extremely useful nowadays, it is not generally available in
a resource-limited public hospital or emergency situation.
2
As a result, anatomical landmarks are still important
for neurosurgeons, especially when performing initial
burr hole placement.
1-9
e temporal bone is known
for its complexity with various bony landmarkssuch
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as the squamosal suture,parietomastoidsuture,and
supramastoidcrest. ere has been controversy in previous
anatomical studies regarding the best bony landmark of
TSSJ in which the intersection between the squamosal
andparietomastoidsutures and the intersection between
the squamosal suture andsupramastoidcrest have been
mentioned.
1,6-9
Both intersections have been commonly
used as the bony landmark of TSSJ. Additionally, race-
based dierences of the skull may also aect the surgical
approach and make one bony landmark suitable for
one race but unreliable for another.
10-11
e authors of
this study used dried human skulls to investigate the
relationship between the temporal bone landmarks and
TSSJ.
MATERIALS AND METHODS
One hundred twenty temporal bones from 60
dried ai adult human skulls were evaluated in this
study. e squamosal sutures, parietomastoid sutures,
andsupramastoidcrest were identied at the outer
surface of the skull. On the inner surface, the grooves of
the transverse sinuses and sigmoid sinuses and TSSJ were
identied. For practicality issues, if there was variation in
sutures such as presence of sutural bone causing multiple
sutures, the most conspicuous suture line would be used.
Aer identifying these key structures, a point on the
intersection between the squamosal and parietomastoid
sutures was labeled as the “Apoint”, and the intersection
between the squamosal suture andsupramastoidcrest
was determined to be the “Bpoint”. Both points were then
marked on the outer surface of the skull (Fig1). e A and
B points were then projected onto the inner surface of the
skull and traced via a transillumination technique using
a laser pointer positioned perpendicular to the skull’s
surface (Fig 2). e projected points A and B were then
evaluated according to whether they were situated on
TSSJ (Fig 3). If conrmed, it would be further classied
as the projected points would be positioned exactly at the
anterosuperior margin or other areas of the TSSJ. Also,
the relationship betweenthe anterosuperiormargin of
TSSJ and projectedA and B point was described and a
distance between these landmarks was measured along
the horizontal (X) and vertical (Y) axis.
is study was ethically approved bythe Institutional
Review Board (IRB) at Siriraj Hospital, Mahidol University
(Si 717/2561 (Exempt)).
Statistical analysis
A statistical analysis was performed using PASW
version 22.0 (SPSS, Chicago, IL, USA). Descriptive
statistics were used to investigate characteristics of the
study sample, including median, range, and percentage
for numerical data. Accuracy of the projected A point
and B point for predicting the location of the TSSJ was
analyzed using Pearson’s chi-squared test. A p-value of
less than 0.05 was considered statistically signicant.
Odds ratio (OR) and 95% condence interval (CI) was
estimated from Pearson’s chi-squared test.
Fig 1. Key points on the outer surface
of the skull. (A): “Apoint” (arrow),
dened as the point of intersection
between the squamosal (arrowhead)
and parietomastoid sutures (double
arrowheads); (B): “Bpoint” (arrow),
dened as the point of intersection
between squamosal suture
(arrowhead) and supramastoid crest
(double arrowheads); MP refers to
mastoid process.
Fig 2. Transillumination technique using a laser
pointer perpendicular to the outer surface of the
skull and marking of the projected point (arrow)
on the inner surface of the skull.
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Fig 3. Measurement of distance between the projected
A point (arrow) and the anterosuperior margin of
TSSJ (arrowhead) in vertical (a) and horizontal (b)
directions.
RESULTS
Demographic characteristics
e mean age of the skull specimens was 38 ± 11.1
years (range 18-60 years). Of the sixty skulls, 30 (50%)
were male, and 29 (48.3%) were female. e remaining
skull (1.7%) belonged to an unknown gender.
e relationship between the projected A point and
anterosuperior margin of the TSSJ
e projected A points were located exactly on
the anterosuperior margin of the TSSJ in 38 out of
120 cases (31.7%). In 82 out of 120 cases (68.3%), the
projected A points were not exactly located on the sinus
margin but situated adjacent (either above or below)
theanterosuperiormargin of the TSSJ (Fig 4A).e
distance from the projected A point to the anterosuperior
margin of the TSSJ ranged from -16 to 12 mm (median
0 mm) on the X-axis and -14 to 17 mm (median 0 mm)
on the Y-axis.
e relationship between the projected B point and
anterosuperior margin of the TSSJ
Of the 60 human skulls, one was excluded due to its
unidentiable bilateral supramastoid crest. e projected
B points were located exactly on the anterosuperior
margin of the TSSJ in 60 of the remaining 118 sides
(50.8%). In cases where the projected B points were not
exactly located on the sinus margin, almost all of the
points were situated above the anterosuperior margin
of the TSSJ (57 of 118 sides or 48.3%). e projected B
point of the remaining one side was positioned within
the TSSJ below the anterosuperior margin (Fig 4B). e
distance from the projected B point to the anterosuperior
margin of the TSSJ ranged from -14 to 9 mm (median
0 mm) on the X-axis and -4 to 28 mm (median 0 mm)
on the Y-axis.
Fig 4. e distribution of the projected A (a) and B points (b) related to the location of the transverse sigmoid sinus junction (TSSJ). In both
gures, the intersection between the X- and Y-axis indicate the anterosuperior margin of the TSSJ; SS, sigmoid sinus; TS, transverse sinus.
Supawannawiwat et al.
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Comparison between the accuracy of the projected
A and B points for predicting the location of the
anterosuperior margin of the TSSJ
Sixty out of one-hundred and eighteen sides (50.8%)
of the projected B points were located exactly on the
anterosuperior margin of the TSSJ whereas the projected
A points were located on the anterosuperior margin in
38 of 120 cases (31.7%). is dierence in accuracy was
statistically signicant (p = 0.003, OR 2.2, and 95% CI
=1.3-3.8).
Comparison between the accuracy of A and B points
for predicting the location of the TSSJ (excluding the
anterosuperior margin of the sinus)
e projected A points were located within the
TSSJ in 21 of 120 sides (17.5%) while the projected B
point was located with the TSSJ in only one out of 118
sides (0.8%). is dierence of accuracy was statistically
signicant (p < 0.001, OR 24.8, 95% CI = 3.3-187.8).
e eect of gender on anatomical relationships
When a subgroup analysis with gender was
done, the pattern of relationship between projected
A points, B points and the anterosuperior margin
of the TSSJ was the same as above in both genders.
Between genders, there was no signicant dierence
in relationship between the projected A point, B point
and the anterosuperior margin of the TSSJ (p=0.291 for
A point, p=0.475 for B point)
In both genders, the projected B point was signicantly
more accurate in predicting the location of the TSSJ
(excluding the anterosuperior margin of the sinus) than
the A point (p < 0.001). When predicting the location of
the anterosuperior margin of the TSSJ, the projected B
point was signicantly more accurate than the A point
in females (p=0.004, OR 2.8) but not signicant in males
(p=0.267, OR 1.5).
DISCUSSION
In dealing with surgical lesions in the middle cranial
fossa, temporal craniotomy is the key procedure. However,
it is also used as the major component of more aggressive
skull base approaches such asthe transpetrosalapproach.
In order to perform an eective craniotomy, neurosurgeons
should create an appropriately-sized cranial opening
while avoiding injury of the adjacent major venous
sinuses.
1-10
Since the posterior boundary of temporal
craniotomy is determined using the position of TSSJ, precise
identication of this major venous structure, especially
the anterosuperior margin of the venous junction, is
crucial.Moreover, despite technological advancements
in the neuronavigation system, which helps facilitate
safer and faster surgery
2
, it is not usually available in a
resource-limited public hospital or emergency situation.
erefore, anatomical bony landmarks are still essential
for neurosurgeons in the initial burr hole process before
beginning temporal craniotomy.
e temporal bone is one of the most complex in the
human body as it is full of various anatomicallandmarks,
such as squamosal suture, parietomastoid suture,
supramastoidcrest, etc. e point of intersection between
the squamosal andparietomastoidsutures (A point) and
the point of intersection between the squamosal suture
andsupramastoidcrest (B point) are commonly used as
the surface landmark to help locate TSSJ.
1,6-10
However,
previous anatomical and clinical studies reported
heterogeneous results and no direct comparison between
both the bony landmarks was studied.
UcerlerandGosvanoted that theasterionwas a
reliable bony landmark for TSSJ, however, when itwas
not exactly supercial, it was mostly inferior to TSSJ.
5
is
meant that the asterion was a suitable bony landmark for
posterior cranial fossa approachesbut not for temporal
craniotomy, in which the location of the craniotomy is
superior to TSSJ. RazaandQuinones-Hinojosaproposed a
surgical technique for the extendedretrosigmoidapproach
that includes an initial burr hole that encompasses TSSJ,
however, it was slightlysupratentorial.
2
Despite this, they
did not mention the exact landmark of the burr hole.
Ribas et al. also studied dried human skulls and found
thatthe meeting point between the parietomastoidand
squamous sutures could be easily identied and were
related to the superior margin of the transverse sinus or
oor of the middle cranial fossa.
1
However, this study
did not mention TSSJ directly. Studies by Bozbugaet al and
Day et al used an imaginary line connecting thesquamosal-
parietomastoidsuture junction and mastoid tip to the
identify sigmoid sinus trajectory but they did not directly
study the relationship between this line and the TSSJ.
6-7
Goto and his coworkers also described their
technique for the safe exposure of the sigmoid sinus
in presigmoidapproaches. ey used the intersection
between the supramastoidcrest and squamosal suture as
a landmark for the anterior margin of TSSJ in this large
case series.
9
Li et al. studied anatomical landmarks of
the anterosuperiorpoint of the TSSJ using dried human
skulls. ey compared the location of the squamosal-
parietomastoidsuture junction with their coordinate
system and concluded it was more accurate in localization
of the venous sinus junction.
8
Additionally, a radiological study of cranial surface
landmarks and the venous sinus was conducted bySheng
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and colleagues. ey used computerized tomography
angiography and found that 89% ofthe squamosal-
parietomastoidsuture junctions werelocated superior
and anterior to TSSJ.
10
In our opinion, the use of a
3-dimensional anatomical study is more accurate than
a 2-dimensional radiological study.
ere are also studies showing that how the size,
shape, and structure of the cranium could be dierent
across ethnic groups.
11
ese dierences can be large
enough to aect surgical approaches. Low et al. found that
Europeans had a greater petrous angle than Chinese people
and therefore they recommended a larger craniotomy size
in Europeans.
12
For this reason, our study was specic
to the ai population.Duangthongpon et al. studied
supramastoid crest as a surgical landmark for temporal
craniotomy and found that the supramastoid crest is easy
to identify and safe from injury. However, they did not
compare it with other available landmarks.
13
In our study, three major anatomical landmarks,
including the squamosal and parietomastoid sutures, and
supramastoid crest, were consistently identiable in almost
all specimens. It was only in one specimen (0.8%) that
thesupramastoidcrest could not be identied bilaterally.
Comparing the accuracy of the projected A and B points
in predicting the location of the anterosuperior margin
of the TSSJ, B point was relatively more accurate when
it came to bony landmarks (p = 0.003, OR 2.2, 95% CI
1.3-3.8).
Following the exclusion of the anterosuperior
margin of TSSJ, a signicantly greater proportion of
the projected A point was located within the TSSJ when
compared with the projected B point (p < 0.001, OR
24.8, 95% CI 3.3-187.8). is result implied that using
the B point as a bony landmark for the initial burr hole
in temporal craniotomy carries less risk of major venous
sinus injury.
Moreover, when the projected A and B point were
not located at the anterosuperior margin or within the
TSSJ, the projected B point had a greater accuracy in
localization of initial burr hole and was also able to
avoid inadvertent venous sinus injury. Almost all of the
remaining projected B points were positioned above
theanterosuperiormargin of the TSSJ (48.3%) compared
with the remaining projected A points which were mostly
positioned around (above or below) the anterosuperior
margin of the TSSJ (68.3%).
Our results suggest that when performing temporal
craniotomy in ais, the B point or the intersection
between the squamosal suture andsupramastoidcrest,
is a more reliable temporal bone landmark for localizing
the anterosuperior margin of the TSSJ than the A
point, which is the intersection between the squamosal
andparietomastoidsutures. is is due to the B point
consistent higher accuracy in correct identication, better
predictable relationship, and lower risk of venous sinus
injury.
In order to explain our results, we have to understand
the controversy whether sutural landmarks such as
the asterion are reliable or not.
14-16
In general, sutural
landmarks can be used to “estimate” the location of
major venous sinuses but with caution of individual
variations.
One factor that makes sutural landmarks less
accurate is the presence of additional, irregular sutural
(Wormian) bones which make sutures more varied.
17
is
presence of sutural bone is used to classify the asterion
into type I (with sutural bone) and II.
18-19
However,
the prevalence of type I asterion was round 10-20%
and generally not mentioned in anatomical studies for
surgical purposes.
1-7,9-10,15-16
Since the aim of our study was
practical usage, we used only conspicuous suture lines.
e prevalence of this bone is highest in the lambdoid
suture followed by posteriorly located sutures such as
parieto-mastoid suture.
20
is might explain our result
that show how using 2 sutures is less reliable compared
to the landmark which uses only 1 suture.
ere are three factors that are known to aect skull
size and shape and they may have impacted our results.
e rst factor is race, however, comparing races was
not our goal. As our study population included only
ais, our results are very race-specic and might not
be suitable for other ethnic groups.
e second factor is gender. However, Johnson
et al. showed that the dierence between races is larger
than the dierence between gender within the same race.
Moreover, gender dierences are also unique in each
race.
21
We used equal proportions of both genders in
our study to prevent selection bias. Our results showed
that there were no statistically signicant dierence
between gender regarding relationship between both
skull landmarks and the anterosuperior margin of TSSJ.
Last but not least, the third factor is age. In early
life, the human skull size and shape can change rapidly
but there is minimal growth aer 15 years.
22
In adults,
bone resorption from increasing age can change cranial
morphology.
23
However, this change might not be clinically
signicant. A study of cranial morphometry by Nikita
showed that unlike gender, changes in cranial shape
due to increasing age is not statistically signicant and
therefore it was justiable to pool dierent age groups
in a bioarcheological analyses.
24
Gapert and colleagues
also studied the age eect on sexual dimorphism of adult
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foramen magnum. ey found no signicant age eect,
suggesting that a separation by age is not necessary.
25
From all this evidence, it is reasonable to generalize our
results for ai adults without age stratication.
CONCLUSION
e intersection between the squamosal suture
andsupramastoidcrest serves as a more reliable temporal
bone landmark for localizing the anterosuperior margin
of TSSJ than the intersection between the squamosal and
parietomastoid sutures. Most points with greater reliability
were located at/or superior to the anterosuperior margin
of the TSSJ.
We have no conict of interest to disclose.
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